Ambient air temperature reduction in winter season reduces Coefficient of Performance (COP) of heat pumps. By using of solar thermal energy to increase the evaporator fluid temperature, compressor work decreases and thus the COP of heat pump increases. In this study, the concept of solar assisted heat pump was presented by combination of an unglazed transpired solar collector and an air-to-air heat pump. In this configuration, inlet air of evaporator was preheated by unglazed transpired solar collector. A mathematical model based on first principle method was developed and performance of this system was investigated over a cold season of Kerman city (located in Kerman province in the south of Iran). Results showed that for a 2.5 kW heat pump a 2.5 m(2) collector was required and this collector could decrease the electricity consumption and Co-2 generation of the conventional air-to-air heat pump up to 10%. It was also concluded that, if the collector heats the ambient air less than 0.3 degrees C then the COP of solar assisted heat pump would be less than COP of heat pump.